1. Field of the Invention
This invention relates to the field of magnetic recordings, and more specifically, to magnetic tape units employing one or more rotating heads which record and/or reproduce machine-convertible information while moving in transducing relationship with a magnetic web media or tape, this information being orientated as magnetic domains to form information tracks which extend generally traverse to the longitudinal tape length.
2. Description of the Prior Art
Rotating head magnetic tape units are widely known. In one form a generally cylindrical guide, sometimes referred to as a mandrel or drum, includes a rotating head wheel which carries one or more magnetic read/write heads. The magnetic tape engages the guide means, at one point, makes a helical wrap about at least a portion of said guide means, and exits the guide means at a point which is both axially and circumferentially spaced from the entrance point. The angle of helical wrap can vary in accordance with design choice, but is usually between 180 degrees and 360 degrees. The head wheel rotates so as to sweep its magnetic head or heads traversely across the tape. The angle at which the head enters and exits the tape may vary, in accordance with design choice, from slightly less than 90 degrees to a small angle such as 15 degrees.
Another type of device is one wherein the head wheel is associated with a tape guiding structure which bends the tape traversely into an arcuate shape that conforms to the circumferencial shape of the head wheel. In this device the tape travels in a generally straight line past the head wheel, and is traversely bent by the associated guides as it enters the head wheel area.
The present invention finds utility with either aforementioned type of device, and has been found particularly useful with the helical wrap device.
A major problem encountered in the aforementioned devices is that of maintaining accurate positional alignment (that is registration) between the path of the head wheel carrying the transducing head or heads and skewed oblique data tracks on the media. The skewed condition is particularly true when a data track is written by one tape unit and later read by another tape unit. The skew or misalignment phenomenom between tapes written and read on different tape transport units stems from the fact that the angle at which the oblique data track is recorded on the writing unit differs from the angle at which the reading unit accesses (that is traverses) the recorded track.
Another major problem facing the prior art rotating head devices is the inability of the device to follow a deformed data track and to recover data recorded in the deformed tracks. Due to the lack of track following capabilities, data which are located within deformed tracks are often lost. In an attempt to remedy the problem data tracks are recorded wider than is necessary. The justification for wide tracks (i.e., tracks in the range of from 15 mils to 20 mils) is that if one or more tracks are deformed the data may be recovered. However, wide tracks significantly reduced area data density on the media. This means that on each data cartridge less data is recorded thereon. The current trend in data storage devices is to improve the areal density. One likely solution is to narrow the data tracks within the range of from 2 mils to 8 mils. In order to foster reliability in a narrow track storage system the need arises for a device which is capable of accurately following a data track.
A probable source for the skew problem and deformed data track problem is media deformity. Generally, the media which is used for data recording is flexible and is somewhat sensitive to changes due to the temperature, humidity, pressure, time, warping, etc. Although the recording angle of a data track is within a prescribed range at the time of recordation, any changes in the aforementioned parameters (that is temperature, time, pressure, humidity, etc.) will tend to deform the media (that is the media will expand or contract) generally anisotopically. The deformation changes the angle at which the data track was originally recorded. With a change or deviation in the recordation angle, when an attempt is made to recover the prerecorded data the recording head or heads cannot faithfully trace (i.e., follow) the deformed data tracks and, as such, all or part of the information maybe lost.
A stop gap measure which aims at solving the deformed tracks problem, is to store the recording media under stringent conditions. For example, it is often required that the media be stored in a storage area having prescribed temperature and humidity control. Also, in some situations, the recording media is assigned a useful lifespan at the end of which the data, recorded on the media, has to be transferred to another recording media or re-recorded on the same media. These requirements impose relatively high maintenance costs on a customer while in some cases do not guarantee faithful and/or accurate retrieval of prerecorded data.
Another partial solution is to impose strict manufacturing tolerances for the data recording/reproducing device (DRD). By imposing a strict manufacturing tolerances, it is hoped that the different components in a DRD will fit accurately and minimized misalignment (one of the causes for the track following problem) not only in the same unit but also between different units. However the imposition of strict manufacturing tolerances requirement tends to increase manufacturing cost.
Still another means which the prior art adapts to solve the aforementioned track following problem is a static means as opposed to a dynamic means. Generally, in the prior art rotating head device the magnetic media is guided onto the rotating head via an entry guide and is guided away from the rotating head via an exit guide. In the prior art, skew is corrected by manually adjusting the rotating head and/or manually adjusting entry and/or exit guides. This adjustment effectively changes the angle which the rotating head accesses (i.e. enters) a recorded track on the recording media and effectuates limited skew adjustments. The problem with this type of skew correcting scheme is that it is either done once in the factory prior to shipment or requires the services of a skilled technician to conduct the adjustment. Moreover, this type of adjustment does not allow magnetic transducers to follow a deformed track. In fact, this limited approach does not address the problem of recovering data from deformed data tracks. A more detailed description of a device which uses manual means for limited skew correction is disclosed in U.S. Pat. No. 3,697,676, issued to Orville I. Protas.
In another form of prior art static adjustment, the means for shifting the head wheel and/or the tape guides are automatic. With this type of device, the adjustment is done without external or manual intervention. Although this approach is a significant improvement over the aforementioned manual prior art skew adjustment, its defect is that there are situations wherein it does not afford the recovery of data. For example, in situations wherein a data track is deformed with a bowed trajectory within the intermediate portion of the oblique data track, the data cannot be satisfactorily retrieved using the prior art scheme of adjustment. Since, the adjustment is made at a time prior to the beginning of the head trace across the selected data track.
Probably, one of the main defects with the prior art static skew correction scheme, be it manual or automatic, is that it does not allow for track following which enables the head to follow a skewed and deformed data track or a skewed data track or a deformed data track to recover data therefrom.